Preparation and re-examination of Li₄Ti_4.85Al_0.15O₁₂ as anode material of lithium-ion battery

Spinel-type Al³⁺-doped Li₄Ti₅O₁₂ oxide with the nominal composition of Li₄Ti_4.85Al_0.15O₁₂ was synthesized by a cellulose-assisted glycine-nitrate combustion process at reduced temperatures. X-ray diffraction characterization demonstrated that all Al³⁺ was successfully incorporated into the spinel lattice structure after calcination at 700°C. The Al³⁺ doping did not have obvious effect on the phase formation and phase structure while it led to an increase in surface area and a decrease in crystallite size of the oxide. The discharge capacity, the rate performance and the cycling stability were all slightly improved after the Al³⁺ doping. First discharge capacity ~221 mAh g^-1 was achieved for the as-synthesized Li₄Ti_4.85Al_0.15O₁₂ from calcination at 700°C, higher than 189 mAh g^-1 for the pristine Li₄Ti₅O₁₂ prepared by the same way. Al³⁺ was likely incorporated into both Li⁺ tetrahedral site and Ti⁴⁺ octahedral site with the majority into the Ti⁴⁺ site. Al³⁺ doping into the Li⁺ tetrahedral site increased the reducibility of Ti⁴⁺; consequently comparable electronic conductivity was observed for Li₄Ti₅O₁₂ and Li₄Ti_4.85Al_0.15O₁₂ after the reduction. However, it also induced a decrease of lithium-ion diffusion coefficient and a transition of rate-limiting step of the electrode reaction from electron charge transfer for Li₄Ti₅O₁₂ to Li⁺ diffusion for Li₄Ti_4.85Al_0.15O₁₂. The improved performance from the Al³⁺ doping was mainly attributed to the increased surface area of the oxide.